The invention relates to an R.F. coil system for generating and/or receiving at least substantially homogeneous alternating magnetic fields, notably in nuclear magnetic resonance apparatus. Such a system is arranged about an essentially cylindrical examination space and comprises at least four conductor segments which extend essentially parallel to the longitudinal central axis of the examination space and which are symmetrically oriented at the circumference thereof with respect to at least one plane which extends through the longitudinal central axis, said conductor segments being interconnected by means of connection leads in order to form a closed loop.
European Patent Application No. 83 201 449.2, notably FIG. 2 and the accompanying description, proposes a transmitter/measurement coil which is arranged about an examination space for an object to be examined in a nuclear magnetic resonance tomography apparatus. The proposed transmitter/measurement coil is constructed as a saddle-shaped coil which consists of two coil halves. For the excitation of the coil, the parallel connected coil halves are fed by an R.F. current source. Each of the coil halves consists of two conductor segments which extend in the axial direction of the transmitter/measurement coil as well as of two sections which extend in the circumferential direction of the transmitter/measurement coil. All four elements are connected in series in order to form a ring and are, via supply leads, connected to the R.F. current source at end points of this series connection which form coil connection terminals.
The coil connection terminals are also bridged by a tuning capacitor. The coil and the tuning capacitor together form a parallel resonant circuit which is tuned to resonance at the frequency of the R.F. source. The aim is to improve the R.F. magnetic field efficiency of the transmitter/measurement coil.
As the frequency of the R.F. source increases, the value of the tuning capacitor must be reduced for resonance tuning in accordance with the known formula for resonant circuits. The frequency at which the transmitter/measurement coil itself resonates, without additional tuning capacitor, is referred to as the natural frequency of the coil. The developed length of the conductors of saddle-shaped coils is generally from 15 to 20% of the wavelength of the oscillation at the natural frequency of the coil. The exact value depends on the coil geometry and on stray capacitances of the coil with respect to surrounding conductors. R.F. currents flowing through the coil during operation in the range of the natural frequency exhibit substantial phase shifts along the developed length of the coil conductors. Consequently, the R.F. magnetic field produced by the coil, notably by its axially oriented conductor segments, becomes noticeably inhomogeneous when the operating frequency of the saddle-shaped coil is in the range of the natural frequency. The saddle-shaped coil produces an at least substantially homogeneous R.F. magnetic field only if the frequency of the R.F. source is substantially lower than the natural frequency of the saddle-shaped coil.
Homogeneous R.F. magnetic fields are required, for example, for nuclear magnetic resonance tomography. Therein, the body to be examined, such as a living organism, is introduced into a static, homogeneous primary magnetic field. Because of their nuclear spin, the atomic nuclei have a magnetic moment which is aligned in the primary magnetic field. The degree of alignment increases as the magnetic induction of the primary magnetic field increases.
When a magnetic alternating field acts on the atomic nuclei of the body to be examined in the direction perpendicular to the field lines of the primary magnetic field, the magnetic moments thereof perform a precessional motion which builds up and tips the magnetic moments in the opposite direction with respect to the primary magnetic field. From a quantum mechanical point of view, the magnetic moments are raised to higher energy states by application of energy from the alternating magnetic field; this requires a given amount of energy within a given frequency of the exciting alternating magnetic field. This frequency is referred to as the nuclear magnetic resonance frequency f0 and is linked to the magnetic induction B0 of the primary magnetic field in accordance with the following equation (.times. represents the multiplication sign): EQU f0=g.times.B0.
In the equation, g is the gyromagnetic ratio which depends on the atomic species and which amounts to, for example, 42.58 MHz/T for hydrogen, 17.23 MHz/T for phosphorus, and 11.26 MHz/T for sodium. By a corresponding choice of the frequencies of the alternating magnetic field, the individual atomic species can thus be selectively excited, thus enabling selective measurement of the relaxation signals from the atoms during the decay from their excited state after the alternating magnetic field is switched off.
It has been found that the image quality in nuclear magnetic resonance tomography increases as the magnetic induction of the primary magnetic field increases, because the number of aligned magnetic moments of the atomic nuclei is thus increased proportionally. However, the nuclear magnetic resonance frequencies for the various atomic species are thus also increased. The R.F. coil system (transmitter/receiving coil) which generates the R.F. alternating magnetic field in the examination space and which receives the relaxation signals from the atomic nuclei, must then produce a magnetic field whose homogeneity satisfies the requirements imposed by nuclear magnetic resonance tomography, even in the case of such higher frequencies.
It has been found that sufficiently homogeneous R.F. alternating fields can be generated by means of saddle-shaped coils only if the developed length of the conductors of the saddle-shaped coils amounts to no more than approximately 1/12 of the wavelength at the frequency of the coil currents, that is to say at the nuclear magnetic resonance frequency to be measured. However, because the developed lengths of such a saddle-shaped coil are determined by the dimensions of the body to be examined, saddle-shaped coils are only suitable for generating homogeneous alternating magnetic fields up to a given frequency. For example, in the case of saddle-shaped coils which are proportioned for measurements on the human head and whose axial length and diameter are approximately equal, the developed length amounts to approximately four times the coil diameter. When the latter is fixed at approximately 30 cm, the upper limit for the nuclear magnetic resonance frequency which can still be measured is approximately 20 MHz. For the measurement of the nuclear magnetic resonance of the hydrogen atom, the magnetic induction of the primary magnetic field may then amount to no more than approximately 0.5 T. For a satisfactory image quality in nuclear magnetic resonance tomography and notably also for a good resolution in nuclear magnetic resonance spectroscopy on a living organism, however, magnetic induction values of up to approximately 2 to 2.5 T are desired for the primary magnetic field.
The drawbacks of the known R.F coil system have been described above with reference to nuclear magnetic resonance tomography, but they occur in any arrangement in which at least substantially homogeneous R.F. alternating magnetic fields are to be generated in spaces whose dimensions are no longer small with respect to the wavelength of the R.F. field generated.